This proposal examines the molecular basis of cell specialization and cell-cell communication in the yeast Saccharomyces cerevisiae. In the course of its life cycle yeast exhibits three distinct cellular phenotypes. Two of the cell types, a and alpha, are specialized for mating. They are distinguished by the production of cell-type-specific proteins, for example pheromones and pheromone receptors, that enable them to signal their presence to one another and prepare for mating. There are two broad goals for the work proposed here. First, we want to understand the molecular basis for cell specialization. The differential gene expression that generates the a and alpha cell types is governed by regulatory proteins, alpha1 and alpha2, encoded by the alpha mating-type locus. These proteins work in combination with a general transcription factor, MCM1, to control transcription of alpha-specific and a-specific genes. A remarkably small segment of MCM1 is sufficient to carry out all its known functions. We will mutagenize this segment to identify residues that are required for interaction with co-regulators and for transcription activation. In addition, we will identify functions that interact with a glutamine-rich segment of MCM1 that is also capable of bringing about transcription activation. The second goal is to understand the pathway of response to pheromone. Binding of pheromone to its cognate receptor activates a signal transduction pathway that elicits physiological changes in the responding cell. These changes include arrest of progression through the mitotic cell cycle and an increase in the transcription of genes whose products catalyze mating. Although several steps in the response pathway have been identified, the relationships among the components and how their activities are regulated is not known. We will carry out a systematic search for mutants that exhibit a constitutively active response pathway. We will also identify new pathway components by isolating mutants that cannot carry out particular aspects of pheromone response. For example, we will select for mutants that cannot undergo cell cycle arrest but are normal for transcription induction. We will order components of the pathway by double mutant analysis, using mutants identified in this project as well as existing mutants. We will determine how the activity of a particular pathway component, the protein kinase STE11, is regulated. This effort will be guided by the properties of dominant, constitutive STE11 alleles recently isolated by us. Finally, we will identify functions that operate at the receptor to attenuate the pheromone-generated signal and therefore allow mitotic growth to resume.